Semiconductor growth systems may be used to form a number of different semiconductor materials, such as nitride-based semiconductors. Examples of nitride-based semiconductors include gallium nitride and its alloys. Nitride-based semiconductors may be formed using semiconductor growth systems, such as those associated with metallorganic chemical vapor deposition (MOCVD) and hydride (or halide) vapor phase epitaxy (HVPE). More information regarding MOCVD and HVPE growth systems may be found in U.S. Patent Application Nos. 20060076559 and 20060118513, as well as in M. A. Mastro et al., “Influence of polarity on GaN thermal Stability”, Journal of Crystal Growth, Vol. 274, Pages 38-46 (2005) and M. A. Mastro et al., “Thermal stability of MOCVD and HVPE GaN layers in H2, HCl, NH3 and N2”, Physica Status Solidi (a), Vol. 188, No. 1, Pages 467-471 (2001).
Semiconductor growth systems generally include a reaction chamber in which a layer of the nitride-based semiconductor is formed on a substrate in response to the introduction of one or more gases therein. The semiconductor growth system typically includes gas lines coupled to the reaction chamber so that the gases may be flowed into it. Further, the semiconductor growth system often includes a heater which heats the gases so they decompose into one or more chemical species. The heater also heats the substrate to a growth temperature so the chemical species interact and form the semiconductor layer on the substrate.
The gases generally depend on the growth system being used to form the layer of semiconductor material. For example, HVPE utilizes hydride precursor gases, such as arsine (AsH3), ammonia (NH3) and phosphine (PH3), as well as additional precursor gases. Further, MOCVD utilizes gases, such as trimethylgallium (Ga(CH3)3), trimethylaluminum (Al2(CH3)6) and trimethylindium (In(CH3)3), as well as additional precursor gases. One type of precursor gas is a metal-chloride based precursor gas, such as gallium chloride (GaCl), aluminum chloride (AlCl) and indium chloride (InCl). Precursor gases of GaCl, AlCl and InCl are used to form gallium nitride, aluminum nitride and indium nitride, respectively, and alloys thereof. Precursor gases may be provided, such as by forming them in-situ by flowing a chlorine containing gas, such as hydrogen chloride (HCl), over a molten metal of gallium, aluminum or indium. Metal-chloride precursor gases may also be provided ex-situ by introducing gallium trichloride, indium trichloride and aluminum trichloride into the reaction chamber.
The precursor gas may include different metal-chloride molecules. The metal-chloride molecule present depends on factors, such as pressure, temperature, as well as the type and amount of gases present in the reaction chamber. For example, the metal-chloride molecules may be in the form of a monochloride molecule (GaCl, AlCl, InCl), chloride monomer (GaCl3, AlCl3, InCl3) and chloride dimer (Ga2Cl6, Al2Cl6, In2Cl6).
However, when it is desired to form gallium nitride, it is generally desirable for the gas to include monochloride molecules instead of chloride monomers and dimers because gallium nitride is more likely to be formed. Gallium nitride is less likely to be formed when the gas includes gallium chloride monomers and/or dimers because gallium monomers and dimers are more likely to gas phase interact. When the gallium chloride monomers and dimers gas phase interact, they are less likely to react with ammonia to form gallium nitride. Thus, the gallium nitride growth rate and the efficiency at which the precursor gases are used both increase in response to having the gas include more monochloride molecules, and fewer monomer and dimer molecules.
The metal-chloride molecules often change from one type of molecule to another in response to semiconductor growth parameters, such as changes in pressure, temperature, as well as the type and amount of gases present in the reaction chamber. For example, gallium chloride at low temperatures forms a chloride dimer (Ga2Cl6) that decomposes into a chloride monomer (GaCl3) at an intermediate temperature, and into gallium monochloride (GaCl) at higher temperatures. Aluminum chloride and indium chloride molecules change in the same way as gallium chloride molecules in response to temperature changes. The change of the metal-, chloride molecules from one type of molecule to another is not easily controllable. Hence, it is desirable to determine and control the type and amount of the metal-chloride gas molecules in a gas in response to changing the growth parameters.
There have been several attempts to determine the type and amount of metal-chloride gas molecules in a gas. More information may be found in U.S. Pat. Nos. 5,070,246 and 5,652,431, as well as U.S. Patent Application No. 20080124453, the contents of all of which are incorporated herein by reference. Information may also be found in an article by Laubengayer et al., which was published in the Journal of the American Chemical Society, page 1578, volume 62 in 1940 and in an article by Kuniya et al., which was published in the Journal of Crystal Growth, page 385, volume 28 in 1975.